Presently known fluid pressure brake systems may typically comprise, in addition to a plurality of service brake circuits, other auxiliary circuits such as an auxiliary or safety brake circuit and a fluid pressure circuit for attendant accessories of the system. Safety valve devices are provided for controlling the charging of the several circuits from a fluid pressure source in a certain desired order such that charging fluid pressure is supplied to the auxiliary circuits only after the service brake circuits have been changed, which means that the vehicle may be set in motion only after pressure has been adequately built up in at least one of the service brake circuits and sufficiently in the auxiliary safety brake circuit for releasing the spring-loaded brake cylinders.
Pre-set fluid pressure responsive supply valves are interosed in each of the circuits between the source and the circuit itself, said supply valves being pre-set to open to respective pressure to provide the priority of charging of the respective circuits, as above noted. These supply valves, of course, are subject on the supply side to pressure of the source and on the delivery side to the prevailing pressure in the respective circuit. Consequently, if a fluid pressure leak develops in one of the circuits as, for example, the one with the supply valve having the lowest opening pressure setting, it might be possible that even the intact circuits could not be charged since the supply pressure would simply escape to atmosphere via the supply valve of defective circuit.
The above-discussed shortcoming of presently known brake systems may be overcome by providing one-way bypassing means in parallel relation to each supply valve, thus making it possible to provide pressure to the intact circuits. But since the several circuits are all parallel to each other, additional check valves must be provided in each circuit to insure proper priority of charging of the several circuits. Such additional structure entails further excessive costs.